1. Field of the Invention
The present invention generally relates to an aluminum/ceramic bonding substrate wherein an aluminum member is bonded to a ceramic substrate. More specifically, the invention relates to an aluminum/ceramic bonding substrate used as an insulating substrate for a power module or the like.
2. Description of the Prior Art
In recent years, power modules are used for controlling heavy-current for electric vehicles, electric railcars, machine tools and so forth. In such a power module, a plurality of semiconductor chips are mainly mounted on a substrate to derive heavy-current. Therefore, substrates for fixing the semiconductor chips thereto must have a high electric insulation performance and a high radiating performance. In addition, the substrates must have a high reliability and a high durability since they are mounted on vehicles, electric railcars and so forth.
As one of indexes for evaluating such reliability and durability, there is a heat cycle test. In such a heat cycle test, thermal shocks are applied on a sample by changing temperature in a short time between the lowest and highest temperatures which are considered in a working environment. Even if such a thermal shock cycle is repeated, it is required to prevent the deterioration of the insulating and radiating performances of the substrate.
If a metal/ceramic bonding substrate having a metal member bonded to a ceramic substrate is used as an insulating substrate for fixing semiconductor chips thereto, when a heat cycle is applied on the bonding substrate, stress occurs on the bonding interface between the ceramic substrate and the metal member due to a difference in coefficient of thermal expansion between the metal and ceramic, so that there is the possibility that the ceramic substrate is finally broken. However, if an aluminum member is used as the metal member, the concentration of stress on the ceramic substrate is relaxed by easily deformable characteristics of aluminum, so that the reliability to the heat cycle is remarkably improved in comparison with a case where a copper member or the like is used as the metal member. For that reason, an aluminum/ceramic bonding substrate having an aluminum member bonded to a ceramic substrate is used as an insulating substrate for high reliable power modules for vehicles and electric railcars.
In a typical conventional power module, a metal/ceramic insulating substrate is fixed to one side of a base plate of a metal plate or composite material by soldering, and semiconductor chips are fixed to the metal/ceramic insulating substrate by soldering. In addition, a radiating fin or cooling jacket of a metal is mounted on the other side (reverse) of the base plate via a thermal conductive grease by means of screws or the like. However, if an aluminum/ceramic insulating substrate is used as the insulating substrate for the power module, there is the possibility that cracks may be produced in the solder layer to remarkably deteriorate the radiating performance of the power module to increase the temperature of the semiconductor chips to finally break the semiconductor chips. In order to prevent this, it is proposed that the Vickers hardness of an aluminum alloy member of an aluminum/ceramic bonding substrate is set to be 25 or more and less than 40 (see, e.g., Japanese Patent Laid-Open No. 2002-329814).
It is studied that semiconductor chips of compound semiconductors, such as SiC, AlGaN and GaN, resistant to a high temperature are used as semiconductor chips for power modules for use in vehicles and electric railcars in future. In such a case, it is studied that power modules having no cooling systems are used to reduce costs. As an insulating substrate for such power modules, it is required to provide an insulating substrate which has an electric insulating function, a circuit function and a cooling function and which is highly reliable to high-temperature heat cycles, particularly resistant to high-temperature heat cycles in each of which the substrate is sequentially held at −50° C. for 30 minutes, at 25° C. for 10 minutes, and at 300° C. for 30 minutes.
The aluminum/ceramic bonding substrate proposed in Japanese Patent Laid-Open No. 2002-329814 can be resistant to 3000 heat cycles or more, in each of which the substrate is sequentially held at −40° C. for 30 minutes, at 25° C. for 10 minutes, and at 125° C. for 30 minutes. However, this substrate can be resistant to only about 1000 high-temperature heat cycles on the above described conditions. It is considered that the reason for this is that the aluminum alloy of the aluminum/ceramic bonding substrate proposed in Japanese Patent Laid-Open No. 2002-329814 is soft to be broken although the ceramic substrate is not broken, so that the fatigue failure of the aluminum alloy occurs. That is, it is considered that the fatigue failure of the aluminum alloy of the aluminum/ceramic bonding substrate proposed in Japanese Patent Laid-Open No. 2002-329814 does not occur after 3000 heat cycles or more on the above described conventional heat cycle conditions, but the fatigue failure of the aluminum alloy occurs after about 1000 high-temperature heat cycles on the above described high-temperature heat cycle conditions.